Intra-annual dynamic of NSCs
The values and the modelled intra-annual dynamics of NSCs (total sugars, starch and soluble sugars content) measured in the beech stemwood are reported in Fig. 3 (panels a, b and c), while the intra-annual statistical differences and the multiple comparisons for 2016 and 2017 are shown in Fig. S4.
Dynamic of NSCs showed polynomial equation patterns at different grades, with R2 ranging from 0.64 to 0.93 (Table 1). Comparing the modelled NSCs intra-annual dynamics and stand phenology, an increase in total NSCs is observed from the bud break to the beginning of green-up phase, due to the increasing starch content notwithstanding the decrease of soluble sugars. During the period between the onset and the middle of the maximum vegetative season, total NSCs content decreased due to starch reduction, while the amount of soluble sugars remained unchanged. In the late summer, both starch and soluble sugars increased until the end of the vegetative season, determining an increase of total sugars content (Fig. 3a). At the beginning of dormancy phase, a decrease of total NSCs was recorded, driven by a severe decrease of starch although associated with a simultaneous increase in soluble sugars.
In 2016, during the leafless period soluble sugars content significantly decreased from 8.15 ± 0.58 mg g DW–1to the lowest value (5.25 ± 0.43 mg gDW–1), which was measured close to the build-up of the new photosynthetic apparatus. Before the senescence, a peak of soluble sugars content was observed (241 vs 215 Day of the Year, DOY), while during the dormancy phase the maximum soluble sugars content (16.75 ± 0.50 mg g DW–1) was assessed. A significant increment of starch content was observed from the beginning of the vegetative period (13.35 ± 1.89 mg gDW–1) until after the canopy destruction (24.80 ± 0.20 mg g DW–1). Subsequently, a decline in the starch content was observed before a second seasonal peak (DOY 241), while the lowest starch content was recorded during the dormancy phase (9.37 ± 0.78 mg gDW–1). In 2016, two seasonal peaks of stemwood total sugars content were observed, with a significant increase (110 vs 151 DOY) and decrease (151 vs 215 DOY) in spring and summer, respectively. At the end of the vegetative season (241 vs 215 DOY) a significant increment of total sugars was recorded.
In 2017, during spring and early summer a significant decrement of stemwood content of soluble sugars until the lowest seasonal value (6.16 ± 0.41 mg g DW–1, DOY 186) was measured. A significant increment of soluble sugars content was observed between July and August (218 vs 241 DOY), before reaching the maximum value of 15.27 ± 0.76 mg g DW–1during the senescence. In spring and early summer 2017, stemwood starch content significantly increased from 11.57 ± 1.51 mg gDW–1 to 20.26 ± 1.92 mg gDW–1. At the end of August, a significant decrement of starch content was observed (12.76 ± 1.92 mg gDW–1, DOY 241). Stemwood total sugars were lower at the beginning of the vegetative period (23.51 ± 2.38 mg gDW–1) than during the senescence, when the maximum value was reached (31.94 ± 3.03 mg gDW–1).
Results from the linear mixed model showed that sampling dates account for 52, 43, and 80 % of the total variance of starch, total NSCs and soluble sugars, respectively. Meanwhile, trees explain 8, 11, 1% of the total variance of concentration of starch, total NSCs and soluble sugars, respectively.
Although the seasonal trends of stemwood carbohydrates accumulation in 2016 and 2017, were similar to the modelled NSCs dynamic recorded in the reference period, some substantial differences can be observed. After the leaf re-sprouting in 2016, starch content decreased considerably, reaching a value lower than the modelled reference value at the site. In August 2016, soluble sugars increased until leaves senescence. After that, a reduction of starch and consequently of the total carbohydrate reserves was observed. The lower amount of storage carbohydrates reached in 2016, directly affected the starch amount recorded during the first part of the vegetative season in 2017. At the beginning of August 2017, although the starch content was lower than the modelled value of the site, a refilling of total carbohydrate reserves was observed. The drought stress event of August 2017 strongly affected the composition of carbohydrate reserves due to a severe starch hydrolysis, leading to a decrease of starch content of about 35% and a parallel increase of soluble sugars. During the late phase of the 2017 vegetative season, the carbohydrates pattern returned close to the modelled intra annual dynamic, although during the dormancy stemwood starch content was lower than the modelled value of the site.